<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
<style>
    #warpper{
        position: relative;
        height: 500px;
        width:100%;
        background: lightblue;
        border:1px solid red;
    }
    #header{
        position: static;
        height:20px;
        width:100%;
        background: red;
    }
    #inner{
        position: absolute;
        top:0;
        height:200px;
        width:100%;
        background: lightgreen;
    }
</style>
<div style="height: 200px"></div>
<div id="warpper">
    <div id="header"></div>
    <div id="inner"></div>
</div>

</body>
</html>